There have been heretofore known a traffic light detecting device for detecting a traffic light from an image captured by a camera (see Japanese Patent Application Publication No. 2005-301518). According to Japanese Patent Application Publication No. 2005-301518, a portion indicating a color of a signal lamp is extracted from the image, the circularity indicating how close to a perfect circle the extracted portion is is calculated, and a portion having a higher circularity is detected as a candidate for the signal lamp.
To be detected as a candidate for a signal lamp, the extracted portion needs to have an image size large enough for the circularity to be determined. Thus, the technique in Japanese Patent Application Publication No. 2005-301518 has a difficulty in accurately detecting a distant traffic light the image size of which is too small to determine the circularity.